1. Field of the Invention
The present invention relates to fabrics made of synthetic materials and particularly, but not exclusively, for use in dryer sections of papermaking machines involving high temperature.
2. Description of Prior Art
Increasingly dryer fabrics are being manufactured from monofilament strands because such fabrics are easier to keep clean, thus retaining their drying efficiency, and because they are essentially non-absorptive. These are normally of woven construction, but in recent years an alternative non-woven construction is becoming popular--the so-called "spiral fabrics"--which are assembled from a multiplicity of helical coils connected together by inserted hinge pins. German patent DE No. 2419751 and U.S. Pat. No. 4,481,079 describe this type of fabric. The predominant material used in such fabrics, whether woven or spiral, is polyester, with polyamides used less frequently. Unfortunately, both of these classes of materials degrade at high temperature, a shortcoming which precludes their use in high temperature applications on paper machines. High temperature applications are ones that result in operating temperatures of about 150.degree. C. or above.
In some applications, large temperature differences are established in the fabrics. Where this happens, the degradation will be the most severe where the temperature is the highest. One example is where the most severe degradation occurs at the edges of the fabric which extend beyond the paper web and accordingly are in direct contact with the heated dryer cans. Another example is in paper machine dryer sections where a single felted configuration is used. This configuration is described in FIG. 2 of U.S. Pat. No. 4,290,209. In such cases, one surface of the fabric is in direct contact with the heated dryer cans as it progresses through the dryer section. The most severe degradation of these fabrics occurs at that surface which is in contact with the dryer cans.
Also, chemical contamination such as oil and grease or chemicals used in papermaking can cause increased degradation. This contamination is often localized. In the case of oil and grease, it is generally located at the edges of the fabric. In the case of papermaking chemicals, it is often localized to the surface of the fabric that is in contact with the paper web.
Some manufacturers of woven fabrics have resorted to NOMEX or KEVLAR in order to cope with the extreme conditions prevalent in such high temperature applications. Because neither polymer is melt-extrudable, monofilaments made from them are not practicable and so these materials are employed in the form of composite multifilaments, often resin coated. U.S. Pat. No. 4,159,618 teaches such a monofilament-like composite strand for this purpose, but even these composites are deficient in that they lose tensile strength when exposed to moist or dry heat (see Tables 1-3 in U.S. Pat. No. 4,159,618).
Considering now another property of dryer fabrics, it is highly desirable that such fabrics be distortion resistant, that is, have inherent dimensional stability and retain this property so as to resist skewing throughout their life on the paper machine. Woven fabrics made with monofilament warp of round cross-section and conventional materials such as polyester, while having the desirable advantages of running clean and of non-absorptivity already mentioned, are generally deficient in distortion resistance because of the minimal interlocking contact at the warp and weft cross-overs dictated by the geometry of the respective strands. U.S. Pat. No. 4,290,209 discloses the use of rectangular cross-section warp strands having a flattening ratio of about 2:1, whereby the resulting fabric acquires superior properties of distortion resistance and surface smoothness, along with more desirable permeability and elastic modulus. None of these improved fabrics, however, are suitable for high temperature applications, again because of the inherent tendency of the polymers normally used to degrade and lose strength.
U.S. Pat. No. 4,359,501 discloses an industrial fabric, for use in applications involving elevated temperatures, comprised of melt-extrudable polyaryletherketone monofilament strands. This material, however, suffers the major disadvantage of being so costly that the woven end product is not economically attractive to the specific paper mill end-users already identified.
The present invention is directed towards solving these problems.